Nuclear magnetic resonance (NMR) studies certain nuclei by aligning them with an applied constant magnetic field (B0) and perturbing this alignment using an alternating magnetic field (B1), orthogonal to the constant magnetic field. The resulting response to the perturbing magnetic field is the phenomenon that is exploited in magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI).
In contrast to NMR, nuclear quadrupole resonance (NQR) transitions of nuclei can be detected in the absence of a magnetic field, and for this reason NQR spectroscopy is referred to as “zero Field NMR.” The NQR resonance is related to the interaction of an electric field gradient (EFG) with the quadrupole moment of the nuclear charge distribution. Because the EFG at the location of a nucleus in a given substance is determined primarily by the valence electrons involved in the particular bond with other nearby nuclei, the NQR frequency at which transitions occur is unique for a given substance. A particular NQR frequency in a compound or crystal is proportional to the product of the nuclear quadrupole moment, a property of the nucleus, and the EFG in the neighborhood of the nucleus. It is this product which is termed the nuclear quadrupole coupling constant for a given isotope in a material and can be found in tables of known NQR transitions.
One application of NQR is for the detection of non-metallic chemical compounds hidden in an opaque medium, such as covered by walls or buried in the ground. Such compounds are of interest in the search for contraband, such as narcotics or explosives. In particular the chemical state of Nitrogen (14N) is detectable by NQR and has a NQR frequency response signature that uniquely indicates many such compounds.
However, current NQR techniques are plagued by several problems. Increased penetration of a medium in which the target material is hidden involves lower frequencies that require a larger antenna or greater power inefficiencies. Also, the antennas suffer from loading effects by water in the medium and by any nearby broadcasting antenna. The above problems are shared by ground penetrating radar (GPR) as well. Furthermore, NQR suffers from the added complication of listening on a receiving antenna for a frequency signature with a power level that is orders of magnitude below the power level of the interrogating electromagnetic pulse emitted by a transmitting antenna. To accommodate the low return power, many current NQR systems use bulky and expensive detectors, such as a superconducting quantum interference device (SQUID), or use processing steps, such as pulse sequencing and background subtraction, to enhance a weak return signal.